This invention relates in general to antennas for radiating electromagnetic signals. More particularly, the invention relates to helical antennas for portable radios and other communications equipment.
One conventional helical antenna is shown in FIG. 1 as antenna 10. Antenna 10 is a simple quarter wave (.lambda./4) structure consisting of a quarter wave helical element 15 coupled to a radio frequency (RF) output 20 mounted on radio case 25. .lambda. is defined as the wavelength corresponding to the desired center frequency of antenna 10. Functionally, such a structure may be viewed as an asymmetric dipole in which the helical element 15 is one element and radio case 25 is the other element. In one typical configuration of the antenna of FIG. 1, helical element 15 contributes approximately 6 ohms to the impedance of the antenna and radio case 25 contributes approximately 44 ohms to the antenna impedance. The impedance contributed by radio case 25 includes both the radiation resistance of case 25 and the ohmic losses due to RF currents in and on case 25. Thus, the overall impedance of a quarter wave helical element situated above a radio, such as in the example of antenna 10 above radio case 25, is approximately 50 ohms. This 50 ohm antenna impedance is conveniently matched with the 50 ohm impedance of radio output 20. In this conventional quarter wave helical antenna, there is a direct physical connection between helical element 15 and output 20 of the radio. Unfortunately, with this approach, relatively high RF currents flow in radio case 25. Thus, when the radio user touches the radio case 25 while operating the radio, the user dissipates these RF currents so as to undesirably decrease the strength of the radiated signal.
Those skilled in the art appreciate that it is generally desirable to have high RF currents in the antenna of a portable radio in order to transmit the strongest signal possible. One way to excite such high currents is with a resonant half-wave helical antenna 30 as shown in FIG. 2. In antenna 30 a quarter wave transmission line transformer 35 is used to directly couple the radio RF output 20 to one end of a half wave (.lambda./2) resonant element 40. Unfortunately, although high levels of RF current are generated in such an antenna, a large RF current is still excited in radio case 25. Thus, as in the case of the quarter wave helical antenna of FIG. 1, the performance of antenna 30 is degraded when the user touches the radio case 25.